P0684

Glow Plug Control Module 1 to PCM Communication Circuit Range/Performance

Q: Is it safe to drive with P0684?

Short term usually yes, but cold-start pre-heating may not work correctly. Fix it before cold weather to avoid a hard or failed start.

Q: Can I just clear P0684?

It clears, but returns whenever the communication fault recurs. The wiring between the two modules needs inspection first.

Glow plugs (diesel) Generic OBD-II High

What it means

P0684 is a diesel code meaning the PCM sees a range or performance problem in the communication circuit between it and the glow plug control module — the two are talking, but the data looks implausible or intermittent. Because that link governs cold-start pre-heating, a fault here can make cold starts hard and smoky. It usually points at the wiring, connectors, or the glow-plug module rather than the engine.

High

Can I keep driving? The engine usually still runs, but the PCM and glow-plug module are not exchanging data reliably, so cold-start pre-heating may be off until it is fixed.

Real-world scan data

Seen in

scans in the past year

Found in

<0.1%

of scans

<0.1%

of scans that found faults

Figures reflect only diagnostic scans recorded through our own apps and tools. Cars are typically scanned because a symptom or warning light prompted it, so these counts describe our scan records — not the incidence of faults in any model, and not the quality, reliability, or safety of any vehicle. No comparison between manufacturers or models is implied. All trademarks belong to their respective owners; no affiliation or endorsement is implied.

Common symptoms

What you'll notice

Check-engine light on (often with a glow-plug warning light)
Hard or slow cold starts
Rough running and white smoke on a cold start
Glow-plug indicator behaving oddly (staying on or not lighting)

Common causes

Most likely first

Damaged or corroded wiring in the communication line between the glow-plug module and PCMLikely
Poor connector or ground at the glow plug control moduleCommon
Faulty glow plug control module sending implausible dataCommon
Failed PCMLess often

How to diagnose & fix

Work through it in order

Scan and record freeze-frame data; note any companion glow-plug codes.
Inspect the communication wiring and connectors between the glow-plug control module and PCM for corrosion, damage, or loose pins.
Verify the module's power and ground feeds are clean and tight.
If wiring and feeds check out, replace the glow plug control module; consider the PCM only after the module and harness are ruled out.

FAQ

Quick answers

Is it safe to drive with P0684?+

Short term usually yes, but cold-start pre-heating may not work correctly. Fix it before cold weather to avoid a hard or failed start.

Can I just clear P0684?+

It clears, but returns whenever the communication fault recurs. The wiring between the two modules needs inspection first.

Read & clear P0684 yourself

Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.

▶ Get it on Google Play

✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX